Multiattack

One Claw attack and one Shell Bash attack.

Claw

Melee Weapon Attack: +5 to hit, 5 ft., one target, 6 (1d6+3) bludgeoning damage and the target is grappled (escape DC 13). The crab has two claws each of which can grapple only one target.

Shell Bash

Melee Weapon Attack: +5 to hit, 5 ft., one target, 5 (1d4+3) bludgeoning damage + 3 (1d6) slashing damage.